linux-kernel - Re: [RFC 3/5] dma-mapping: Enable global non-coherent pool support for RISC-V

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Date:   Tue, 27 Jul 2021 10:52:44 +0200
From:   Christoph Hellwig <hch@....de>
To:     Atish Patra <atishp@...shpatra.org>
Cc:     Christoph Hellwig <hch@....de>, Atish Patra <atish.patra@....com>,
        devicetree <devicetree@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Tobias Klauser <tklauser@...tanz.ch>,
        Robin Murphy <robin.murphy@....com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        iommu@...ts.linux-foundation.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Frank Rowand <frowand.list@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [RFC 3/5] dma-mapping: Enable global non-coherent pool support
 for RISC-V

On Mon, Jul 26, 2021 at 03:47:54PM -0700, Atish Patra wrote:
> arch_dma_set_uncached works as well in this case. However, mips,
> niops2 & xtensa uses a
> fixed (via config) value for the offset. Similar approach can't be
> used here because the platform specific
> offset value has to be determined at runtime so that a single kernel
> image can boot on all platforms.

Nothing in the interface requires a fixed offset.  And using the offset
has one enormous advantage in that there is no need to declare a
statically sized pool - allocations are fully dynamic.  And any kind of
fixed pool tends to cause huge problems.

> 1. a new DT property so that arch specific code is aware of the
> non-cacheable window offset.

Yes.

> individual device if a per-device non-cacheable
>    window support is required in future. As of now, the beagleV memory

If you require a per-device noncachable area you can use the per-device
coherent pools.  But why would you want that?

> region lies in 0x10_0000_00000 - x17_FFFF_FFFF
>    which is mapped to start of DRAM 0x80000000. All of the
> non-coherent devices can do 32bit DMA only.

Adjust ZONE_DMA32 so that it takes the uncached offset into account.

> > > -     mem = dma_init_coherent_memory(phys_addr, phys_addr, size, true);
> > > +     if (phys_addr == device_addr)
> > > +             mem = dma_init_coherent_memory(phys_addr, device_addr, size, true);
> > > +     else
> > > +             mem = dma_init_coherent_memory(phys_addr, device_addr, size, false);
> >
> > Nak.  The phys_addr != device_addr support is goign away.  This needs
> 
> ok.
> 
> > to be filled in using dma-ranges property hanging of the struct device.
> 
> struct device is only accessible in rmem_dma_device_init. I couldn't
> find a proper way to access it during
> dma_reserved_default_memory setup under global pool.
> 
> Does that mean we should use a per-device memory pool instead of a
> global non-coherent pool ?

Indeed, that would be a problem in this case.  But if we can just
use the uncached offset directly I think everything will be much
simpler.